The present invention relates to the coating arts, particularly those wherein the coating is formed by vacuum vapor deposition.
The oxidation and sulfidation erosion resistance of the nickel-base and cobalt-base superalloys may be markedly increased through the use of a coating thereover which consists essentially of a cobalt alloy containing chromium, aluminum and yttrium particularly at the composition, by weight, of 15- 40 percent chromium, 10- 25 percent aluminum, 0.01- 5 percent yttrium, balance cobalt (CoCrAlY). This coating alloy is discussed in detail in the U.S. Pat. No. 3,676,085, of common assignee herewith, and whose teachings are incorporated herein by reference.
The most common way of generating the coatings is by vacuum vapor deposition. The surfaces to be coated are cleaned free of dirt and other objectionable foreign matter and are conditioned by abrasive blasting. The coating is achieved by vapor deposition from a molten pool of the coating material held in a vacuum chamber at 10.sup.-.sup.4 ton or better. The ingot being melted to form the molten pool has essentially the same chemistry as that of the desired coating. Melting may be accomplished by electron beam heating. The vapors generated are deposited on the surfaces to be coated in the chamber.
One of the problems uncovered in the deposition process has been that oxide inclusions, such as alumina, formed in the formation of the coating ingot rise to the surface of the molten pool during ingot evaporation. Because of the low thermal conductivity of these oxides they become superheated by electron bombardment. A layer of vapor then develops beneath each floating particle causing a depression in the liquid surface. If the liquid then closes around the cavity thus created, an eruption of liquid occurs, ejecting small droplets of liquid from the surface. The net result is the formation of pin holes or droplets on the coating, causing rejection of the part.
The onset of spitting is related to the power supplied to the system and is one of the chief limitations to high evaporation rates.